1. Field of the Invention
The invention relates to the casting of metal strip directly from a melt, and more particularly to the rapid solidification of metal directly from a melt to form substantially continuous metal strip.
2. Description of the Prior Art
U.S. Pat. No. 4,142,571 issued to M. Narasimhan discloses a conventional apparatus and method for rapidly quenching a stream of molten metal to form continuous metal strip. The metal can be cast in an inert atmosphere or a partial vacuum. U.S. Pat. Nos. 3,862,658 issued to J. Bedell and 4,202,404 issued to C. Carlson disclose flexible belts employed to prolong contact of cast metal filament with a quench surface.
The casting of very smooth strip has been difficult with conventional devices because gas pockets entrapped between the quench surface and the molten metal during quenching form gas pocket defects. These defects, along with other factors, cause considerable roughness on the quench surface side as well as the opposite, free surface side of the cast strip. In some cases, the surface defects actually extend through the strip, forming perforations therein.
U.S. Pat. No. 4,154,283 issued to R. Ray et al. discloses that vacuum casting of metal strip reduces the formation of gas pocket defects. The vacuum casting system taught by Ray et al. requires specialized chambers and pumps to produce a low pressure casting atmosphere. In addition, auxiliary means are required to continuously transport the cast strip out of the vacuum chamber. Further, in such a vacuum casting system, the strip tends to weld excessively to the quench surface instead of breaking away as typically happens when casting in an ambient atmosphere.
U.S. Pat. No. 4,301,855 issued to H. Suzuki et al. discloses an apparatus for casting metal ribbon wherein the molten metal is poured from a heated nozzle onto the outer peripheral surface of a rotary roll. A cover encloses the roll surface upstream of the nozzle to provide a chamber, the atmosphere of which is evacuated by a vacuum pump. A heater in the cover heats the roll surface upstream from the nozzle to remove dew droplets and gases from the roll surface. The vacuum chamber lowers the density of the moving gas layer next to the casting roll surface, thereby increasing formation of air pocket depressions in the cast ribbon. The heater helps drive off moisture and adhered gases from the roll surface to further decrease formation of air pocket depressions.
The apparatus disclosed by Suzuki et al. does not pour metal onto the casting surface until that surface has exited the vacuum chamber. By this procedure, complications involved in removing a rapidly advancing ribbon from the vacuum chamber are avoided. The ribbon is actually cast in the open atmosphere, offsetting any potential improvement in ribbon quality.
U.S. Pat. No. 3,861,450 to Mobley, et al. discloses a method and apparatus for making metal filament. A disk-like, heat-extracting member rotates to dip an edge surface thereof into a molten pool, and a non-oxidizing gas is introduced at a critical process region where the moving surface enters the melt. This non-oxidizing gas can be a reducing gas, the combustion of which in the atmosphere yields reducing or non-oxidizing combustion products at the critical process region. In a particular embodiment, a cover composed of carbon or graphite encloses a portion of the disk and reacts with the oxygen adjacent the cover to produce non-oxidizing carbon monoxide and carbon dioxide gases which can then surround the disk portion and the entry region of the melt.
The introduction of non-oxidizing gas, as taught by Mobley, et al., disrupts and replaces an adherent layer of oxidizing gas with the non-oxidizing gas. The controlled introduction of non-oxidizing gas also provides a barrier to prevent particulate solid materials on the melt surface from collecting at the critical process region where the rotating disk would drag the impurities into the melt to the point of initial filament solidification. Finally, the exclusion of oxidizing gas and floating contaminants from the critical region increases the stability of the filament release point from the rotating disk by decreasing the adhesion therebetween and promoting spontaneous release.
Mobley, et al., however, address only the problem of oxidation at the disk surface and in the melt. The flowing stream of non-oxidizing gas taught by Mobley, et al. is still drawn into the molten pool by the viscous drag of the rotating wheel and can separate the melt from the disk edge to momentarily disturb filament formation. The particular advantage provided by Mobley, et al. is that the non-oxidizing gas decreases the oxidation at the actual point of filament formation within the melt pool. Thus, Mobley, et al. fail to minimize the entrainment of gas that could separate and insulate the disk surface from the melt.
U.S. Pat. Nos. 4,282,921 and 4,262,734 issued to H. Liebermann disclose an apparatus and method in which coaxial gas jets are employed to reduce edge defects in rapidly quenched amorphous strips. U.S. Pat. Nos. 4,177,856 and 4,144,926 issued to H. Liebermann disclose a method and apparatus in which a Reynolds number parameter is controlled to reduce edge defects in rapidly quenched amorphous strip. Gas densities and thus Reynolds numbers, are regulated by the use of vacuum and by employing lower molecular weight gases.
Conventional methods, however, have been unable to adequately reduce surface defects in cast metal strip caused by the entrapment of gas pockets. Vacuum casting procedures have afforded some success, but when using vacuum casting, excessive welding of the cast strip to the quench surface and the difficultly of removing the cast strip from the vacuum chamber have resulted in lower yields and increased production costs. As a result, conventional methods have been unable to provide a commercially acceptable process that efficiently produces smooth strip with consistent quality and uniform cross-section.